Hubble Space Telescope Imaging of a Peculiar Stellar Complex in NGC 6946

Elmegreen, B. G., Larsen, S., Efremov, Y. N., Alfaro, E., Battinelli, P., Hodge, P. W., and Richtler, T.
American Astronomical Society, 199th AAS Meeting, #14.01; Bulletin of the American Astronomical Society, Vol. 33, p.1329


The stellar populations in a young spiral arm complex in NGC 6946 are analyzed using images taken with HST WFPC2. The complex is peculiar because of the presence of a young globular cluster (YGC), 15 Myr old with 106 MO, a high density of other young stars, and a sharp, circular shape. The physical dimensions are about the same as for the local Gould Belt, but the stellar density is 1 -- 2 orders of magnitude higher. In addition to the YGC, there are 18 other clusters with luminosities similar to the brightest open clusters in the Milky Way. The entire stellar mass is 107 MO, and the initial cloud was probably 108 MO. Parts of this cloud remain today as dark patches of obscuration. The color-magnitude diagram of individual stars shows a paucity of red supergiants compared to model predictions in the 10-20 Myr age range for a uniform star formation rate. This implies a gap in the dispersed star formation history, with a concentration of star formation in the YGC during this gap. The YGC may have been triggered in a 107 MO residual cloud by pressures from the other stars that formed first. A more dispersed mode of star formation continued after this, presumably in the smaller cloud fragments.